A concept of → star formation
in which → high-mass stars and
→ low-mass stars form in different physical conditions
involving different → molecular clouds.
Following the pioneering suggestion of Herbig (1962), successive investigations
have generally supported the idea that star formation proceeds bimodally with
respect to stellar mass. The star formation rate appears to differ both spatially
and temporally for low mass and → massive stars.
This is of considerable importance
for galactic evolution, since the low-mass stars lock up mass and are long-lived,
low luminosity survivors to the present epoch, whereas massive stars are short-lived,
recycle and enrich interstellar gas, and leave dark remnants while producing
a high luminosity per unit of mass
(Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds.
R. E. Pudritz and M. Fich).

A crucial period in the history of the → Universe,
when the bulk of stars in massive galaxies were
likely formed. Observations of young stars in distant galaxies at different times in
the past have indicated that the → star formation rate
peaked at the → redshift of z ~ 2, some 10 billion years
ago, before declining by a factor of around ten to its
present value (P. Madau & Dickinson, 2014, arXiv:1403.0007).

1) The act or process of forming or the state of being formed, such as
→ star formation.
2) Geology: A laterally continuous rock unit with a distinctive set of
characteristics that make it possible to recognize and map from one
→ outcrop or well to another. The basic rock unit of stratigraphy.

The method of relating a measurement in one
→ reference frame to another moving with a constant velocity
with respect to the first within the → Newtonian mechanics.
The Galilean transformation between the coordinate systems (x,y,z,t) and
(x',y',z',t') is expressed by the relations: x' = x - vt, y' = y,
z' = z. Galilean transformations break down at high velocities and for
electromagnetic phenomena and is superseded by the
→ Lorentz transformations.